The present invention is directed to a method of and apparatus for separating non-magnetic, electrically conductive metals from a mixture of solid particles utilizing an airflow in combination with an alternating magnetic field as the separating means. The airflow is developed in the upward direction through a passageway with the air supply being introduced at the lower inlet end and removed at the upward outlet end. An inlet channel is provided for introducing the mixture of solid particles into the airflow passageway.
In so-called eddy current separation, the materials to be separated are guided between the poles of an alternating magnetic field producer such as on a belt or in a free fall. The eddy currents are induced in the favorably electrically conductive component parts of the mixture to be separated and the eddy currents develop their own magnetic fields directed oppositely to the producer field and, accordingly, the particles are accelerated relative to the rest of the particles in the mixture by electromagnetic forces. With eddy current separation, non-ferrous materials with good electrical conductivity, such as aluminum and copper can be separated from scrap and waste such as automobile scrap, glass waste and the like. In the event ferromagnetic particles are present in the feed, a magnetic separation must be provided before passage through the eddy current separating apparatus, since the ferromagnetic particles would obstruct the working space in the separating apparatus. It is advisable that other preparation stages should precede eddy current separation in order to improve separation efficiency.
Air classification is especially suited for separating lighter particles from heavier particles. The separation is effected, according to the descending speed, in vertical or horizontal air currents. To separate the light and heavy particles the mixture of such particles must be classified within narrow limits to obtain the desired high product grades.
In an air classifier, the airflow directed against the particle flow to be separated, is adjusted so that small (and also large) heavy particles with a form factor deviating sharply from the spherical shape fall downwardly while small and lighter particles are carried upwardly by the airflow. Problems may possibly occur if a portion of relatively large lighter particles are present which fall along with the heavier particles based on their absolute weight and sphere-like form factor. If the strength of the airflow is increased, the larger and lighter particles could be carried along with the other lighter particles, however, at the same time a substantial portion of the heavier particles with corresponding particle or grain size and shape would be carried away with the lighter particles.
Since smaller sized particles of various specific weight can be separated relatively well by air classification, the eddy current separation supposes a minimum particle size when non-ferrous metals are being separated from a mixture of solid particles in a variable magnetic field, because a continuous separating action with a reasonable expenditure of material and energy is only practical, according to this method, for mixtures where the smallest particle size has a diameter of approximately 15 to 20 mm.
A device for eddy current separation in a duct or passageway in which the airflow favors only the loosening of the introduced mixture, is suggested in German Offenlegungsschrift No. 25 09 638. The individual particles in the mixture to be separated arrive in a free fall within the air flow through the gap of an alternating magnetic field producer with the field travelling in a direction transverse to the falling direction. A chief disadvantage of the known device is that the particles in the mixture to be electromagnetically influenced in the direction of the travelling wave must be moved transversely to the flow of the falling particles with the possibility that the various particles collide and interfere with the separation. The portion of the particles inadvertently displaced rises with a mixture introduced into the separator which is not extensively scattered and therefore both products contain the two types of particles due to hindrance and entrainment. The air flowing through the particles should serve to loosen the mixture to be separated in the inlet channel or shaft before the separation zone is reached and not to separate particles of different density. Moreover, a common discharge of purely granular, lighter particles together with coarser lighter particles deflected by the alternating magnetic field producer is not considered. Finally, the individual particles have no defined position and, therefore, can be turned by the alternating magnetic field in a direction in which the field can exert only a relatively small separating force determined by the eddy currents on the individual particles.